Pockets of water may lie deep below Earth’s surface

Small pockets of water exist deep beneath Earth’s surface, according to an analysis of diamonds belched from hundreds of kilometers within our planet. The work, which also identifies a weird form of crystallized water known as ice VII, suggests that material may circulate more freely at some depths within Earth than previously thought. Geophysical models of that flow, which ultimately influences the frequency of earthquakes driven by the scraping of tectonic plates at Earth’s surface, may need to be substantially tweaked, scientists say. Such models also help scientists estimate the long-term rates of heat flow through Earth’s surface and into space.

“These diamonds seem to be returning confirmation, and a few new surprises, of what’s happening deep within Earth,” says Steven Shirey, a geochemist at the Carnegie Institution for Science in Washington, D.C., who was not involved in the study. One of the biggest surprises, he suggests, is evidence for the presence of unbound water at depths below 600 kilometers.

Pure diamonds are made of nothing but carbon, but most contain small impurities that take the form of tiny crystals. These inclusions offer clues about how and where the gems formed, says Oliver Tschauner, a mineralogist at University of Nevada in Las Vegas. In a 2016 study, for example, metal-rich inclusions found in dozens of large, clear diamonds suggested that those gemstones formed in pockets of liquid metal.

Recently, Tschauner and his colleagues analyzed diamonds unearthed at several sites in southern Africa and China. More than a dozen of them contained a new type of inclusion—a distinct form of crystallized water known as ice VII. (Scientists have discovered more than a dozen types of ice crystals, including ice IX—which, unlike Kurt Vonnegut’s fictional ice-nine, doesn’t freeze up the world’s oceans.) Ice VII is well known from lab studies of materials under high pressure, Tschauner says, but the samples he and his colleagues describe are the first known natural samples, the researchers report today in Science. Based on the team’s data, ice VII has been declared a new mineral.

X-rays scattered from water trapped in a diamond (light gray pixels seen near arrow) suggest that watery fluids can be found deep inside Earth.

Tschauner Et al./Science (2018)

The identification of ice inside those diamonds provides scientists with more than a nifty new mineral, Tschauner says. It also suggests that pockets of watery fluids exist at great depths in Earth’s mantle. This water, rather than being chemically bound in rocks in combinations called hydrated minerals, is free-floating and remains a liquid—despite the high temperatures found in the mantle, the layer sandwiched between Earth’s crust and core. The team’s analyses suggest that some of the diamonds they studied formed at depths between 610 and 800 kilometers below Earth’s surface—the first direct evidence of unbonded water at such extreme depths, Tschauner notes. Nevertheless, the new research doesn’t help pin down how large those pockets are or how common they may be.

Alongside the ice VII inclusions were tiny crystals of calcite and various types of salts, Tschauner says. Thus, he and his colleagues contend that the diamonds they analyzed crystallized in pockets of watery, salty fluid at depths well below the level at which scientists had previously identified water unbound to other minerals.

The presence of watery fluids at or below the boundary between the upper and lower mantle could definitely affect how and where heat is generated in the mantle, says Oded Navon, a mantle geochemist at The Hebrew University of Jerusalem. For instance, such watery fluids could more readily carry certain forms of easily dissolved radioactive elements from one part of the mantle to another. That could affect where in the mantle heat-generating radioactive decay occurs, which, in turn, could make the heated areas less viscous and thus prone to flow more readily. All these changes could influence the rates, over the long term, at which heat escapes from Earth’s interior.

Among other things, the varying composition of materials at different layers of the mantle can affect where and how well tectonic slabs that have sunk back into Earth’s interior melt and release their minerals, Tschauner and his team contend. For instance, the density and viscosity of Earth’s interior affect the level at which sinking slabs reach neutral buoyancy, thus stalling their descent. That, in turn, influences where the slabs melt and release the water and other minerals they hold. Overall, the team’s new findings may lead to more accurate models of what’s going on at different depths deep within Earth.